There is considerable interest in providing consumer electronic devices, including smart phones, computers, electronic tablets, and so on, with various means of securing information stored on these devices. Biometric security systems, such as fingerprint recognition systems, are one approach to providing these security features. These systems may not require memorization, or the use of any other device by the user, as security may be based on unique features of the user. Biometric security systems also provide the potential advantage being difficult to ‘crack’ for the same reasons.
Fingerprint recognition systems generally collect fingerprint images and compare those images against a database of known fingerprint information. For example, after a set of fingerprint images for a known authorized user is collected and processed, a user wishing access can be authorized by collecting one or more fingerprint images for that accessing user and comparing these collected images against known fingerprint information for the authorized user. One example of a fingerprint recognition system uses capacitive sensing elements to detect fingerprint images for collection. Such sensors are able to detect electric field differences between ridges and valleys of the fingerprint of a finger in contact with a contact surface of the consumer electronic device adapted for this purpose.
The effectiveness of biometric security systems may be affected by the accuracy with which the unique biometric data on which they are based is able to be detected. In the case of fingerprint identification systems, this means that improved detection resolution may lead to improved security for the secured device. Thus, improving detection resolution is a significant issue in such systems.
Another issue for consumer electronic devices is the availability of space within the device housing, including the amount and the location of this space. Many components contend for this space in the design process. Available surface space is often a particularly limited resource, which leads to designs in which multiple components share surface space. Sometimes this is due to space limitation, sometimes, such as setting a touchscreen over a display stack, this multi-tasking of surface space can lead to additional functionality.
In the case of fingerprint recognition systems that include arrays of capacitive sensing elements to detect the fingerprint images, i.e. capacitive fingerprint sensors, there may be a number of reasons for these components to axially aligned with other components of the consumer electronics device, such as display stacks and touch screens, not the least of which is an interest in avoiding assigning valuable surface space exclusively to an component that may only be used briefly during the process of identifying the user. However, several issues, such as the opacity of the capacitive sensing elements may consign the capacitive fingerprint sensor to a location behind these other components. Thus, the distance from the outer surface of the device, i.e. the cover glass, which serves as the contact surface for the capacitive fingerprint sensor, and the array of capacitive sensing elements may be large enough to affect the resolution of the sensor. This is because such separations may lead to blurring of the electric field and a corresponding loss of resolution of the fingerprint images detected by the capacitive sensor.